Glow discharge device circuit



6, 1 58 N. E. NASH ET AL 2,849,657

GLOW DISCHARGE DEVICE CIRCUIT Filed March 30, 1954 2 Sheets-Sheet 1 Abso/ute /-\\rla Re /naive ii 36 I9 l6 E3 22 I7 x 1* /6 27 I I9 I 28 2/ I I T L Closed, V5 absolute Oken, relative to \4;

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N. Elizabeth Nash, Ted E. Fou Me by Wm M Their Afb'bonney Aug. 26, 1958 Filed March 30, 1954 DC BREAKDOWN m \lou's DC BREAKDOWN m VoLTS DC BREAKDOWN m VOLTS N. E. NASH ET AL 2,849,657

GLOW DISCHARGE DEVICE CIRCUIT 2 Sheets-Sheet 2 No REST TIME 11o -150 -1oo so +100 SHIELD POTENTIAL CHANGE (AV 120 AV CONSTANT 10 100 1000 REST TIME IN MINUTES VWE 1.00

REST TIME IN MmuTEs Inventors:

N. Elizabeth Nash Ted E. FoulKe by M6 Their Attorney United States Patent GLOW DKSEHARGE DEVICE CIRCUIT Nancy Elizabeth Nash, Ziryon, N. C., and Ted E. Foulke,

Cleveland, Ohio, assign-oil's to General Electric Company, a corporation of New York Application March 3th, 1954, Serial No. 419,851

4 Claims. (Cl. Bid-169) This invention relates in general to glow discharge devices having a pair of cold electrodes operating through field emission. It is particularly concerned with circuit arrangements for such devices giving previously unavailable stability and permitting entirely new applications.

The invention finds particular application in miniature glow lamps such as exemplified by the series designated commercially NE and AR. Commercially available miniature glow lamps range in rating upward from watt. The neon glow lamps have starting or breakdown voltage ratings between 60 and 90 on direct current: these ratings can be extended to higher values by varying the gas filling and pressure. It has been observed that there is considerable variation in the breakdown voltage as between lamps of the same type. It has also been observed that the dark breakdown voltage, that is the breakdown voltage when the lamp is located in darkness, is usually considerably higher than the breakdown voltage when incident daylight or radiation from other lamps is striking the glow lamp. However, the most common application of such glow lamps up to the present time has been as visual indicators, for instance in alarms, automatic controls, appliances and wiring devices. For such applications, the performance characteristics are not critical and variations in breakdown potential within a limited range could readily be tolerated.

More recently, such glow lamps have begun to find considerable application as circuit elements in electronic apparatus, for instance in electronic computersand other electronic control circuits. It will be appreciated that in such applications the glow discharge devices are not .used for their light-emitting properties; however, for the sake of convenience, they will be referred to in this specification by their usual designation of glow lamps. For these applications, it is essential that the breakdown voltage of the glow lamp be constant in order to have stability of operation. Since such equipment may operate at times in daylight and at other times in darkness, it might be thought that stability of operation could readily be achieved by enclosing the glow lamp in a light-impervious enclosure. However, such is not the case and it has been disconcerting to observe, until my present invention, that the breakdown potential of even a totally dark-enclosed glow lamp is extremely variable if any prolonged time interval occurs between successive breakdowns. Even -rnore disconcerting has been the discovery that the first pendently of the time interval elapsing between-suc- .cessive breakdowns.

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Yet another object is to provide a circuit for achieving a predetermined breakdown potential in a dark-enclosed glow discharge device of the instant type independently of theprior history of the device as regards the application to it of extraneous, undetermined potentials.

In accordance with the invention, we have discovered that the dark breakdown potential of low pressure glow discharge devices having cold activated electrodes follows a regular pattern depending upon rest time and potential changes on the envelope of the device. The device acts as if a voltage change on either the inside or the outside of its envelope induces a voltage at the cathode which may either aid or buck the potential applied across the electrodes for a subsequent breakdown. The elfect of previous voltage changes, that is the prior signal history of the device or lamp, may be erased by glowing the lamp. After the lamp has been extinguished, a negative change in the voltage applied to the envelope, as on a shield surrounding the envelope, necessitates the application of a higher electrode voltage for breakdown, seemingly by inducing immediately a bucking potential on the cathode. If the lamp is extinguished and allowed to rest, no matter whether or not a voltage change has been applied to the shield and independently of the magnitude of any such change, a terminal breakdown potential is reached after an interval of at most eight hours. The terminal breakdown potential is higher than the normal repetitive or zero change-zero time breakdown potential, that is, the

breakdown potential immediately after extinguishing the lamp and without the application of any change in potential on the shield.

The terminal or plateau breakdown potential can be duplicated without a rest time by applying to the shield a potential change such that the breakdown potential immediately thereafter is equal to the terminal breakdown potential. Since the device is thereby immediately placed at the terminal breakdown potential, there can be no change with respect to time and constancy of breakdown is thereby achieved. Another arrangement for achieving constancy of a breakdown consists in applying to the shield a potential which will prevent the induction of bucking potentials on the electrodes with elapse of time. Under this condition, the breakdown is constant with rest time and equal to the repetitive breakdown of the device, that is the zero change-zero time breakdown.

Further objects and advantages of this invention will become apparent by reference to the following description and accompanying drawings.

The features of novelty which characterize the invention will be pointed out with particularity in the claims annexed to and forming a part of this specification.

In the drawings:

Fig. 1 is aside View of a glow discharge device suitable for the present invention and located in a light-impervious enclosure which has been sectioned to expose the device.

Fig. 2 is a diagram of a circuit suitable for observing the characteristics of the glow discharge device considered herein.

Fig. 3 is a. diagram of a circuit arrangement embodying the invention for achieving the terminal breakdown volt age of the device immediately upon extinguishment.

Fig. 4 is a diagram of another circuit arrangement embodying the invention for stabilizing the breakdown voltage of the device with respect to time.

Fig. 5 shows graphically the variation in breakdown with respect to potential changes applied to the shield.

Fig. 6 shows graphically the variation in breakdown with respect to rest time for various values of shield potential changes.

Fig. 7 shows graphically'the variation in breakdown potential with respect to rest time for various constant ratios of shield voltage to electrode voltage.

Glow device structure Referring to Fig. 1, the glow discharge device 1 illustrated corresponds generally to the-neon glow lamp designated commercially NE-Z and having a rating of watt. The lamp comprises a vitreous tubular envelope 2 in which is mounted a pair of wire electrodes 3, 4 welded on the inner ends of leads 5, 6 sealed into a press '7 at the lower end of the tube. Electrodes 3, 4 may consist of nickel coated with an electron-emissive mixture, for instance a mixture of alkaline earth oxides such as barium and strontium oxides. The leads 5, 6 are preferably of dumet wire, that is, copper-sheathed iron wire which seals readily into glass. The tube contains a filling of an inert gas at a low pressure, for instance a mixture of neon and argon at approximately 130 millimeters of mercury. After filling, the tube is tipped off at its upper end 8. The lamp has been illustrated in Fig. 1 approximately three times full size; in its actual dimensions, the lamp is approximately inch in diameter and /3 inch in over-all length.

The lamp is shown with a close fitting conductive shield 9 to which, is connected a lead wire 11 to allow the application of potential to the shield. The shield may take other forms than that illustrated; for instance, it may consist of metallic foil wrapped around the lamp or of a conductive coating on the glass envelope of the lamp. The lamp and shield are dark enclosed in a lightimpervious enclosure comprising a metal thimble 12 and a cap or cover 13 with suitable openings therethrough for the lead wires 5, 6 and 11.

Test circuit The breakdown characteristics of device 1 may conveniently be investigated in the circuit of Fig. 2. The lamp is surrounded by the shield 9 and is placed within a dark enclosure schematically represented by the dotted circle 12. Electrode 3 of the lamp and also the enclosure 12 are grounded. The other electrode 4 which is to operate as the anode is connected, in series with current limiting resistance 14 and ammeter 15, to switch 16. When the switch is closed, the anode circuit is completed to variable tap 17 on potentiometer 18 across which is connected a unidirectional voltage supply which may conveniently be provided by a battery 19 whereof the negative side is grounded. A voltmeter 21 measures the voltage V applied to anode 4 when switch 16 is closed.

The shield 9 is connected to variable tap 22 on potentiometer 23 connected between ground and a doublethrow switch 24. Switch 24 may be closed to its upper terminal 25 for the application of a fixed potential equal to that of battery 19 across potentiometer 23, or may be closed to its lower terminal 26 for the application of a relative potential determined by the setting of tap 17 on potentiometer 18. When a fixed potential is applied, voltmeter 27, upon the closure of switch 28, measures the potential which the setting of tap 22 applies to shield 9. For a relative potential, the setting of tap 22 determines the ratio which the shield potential V bears to the elec trode potential V and switch 28 is left open.

General breakdown characteristics Using the circuit arrangement of Fig. 2, we have deter-mined that the repetitive value of breakdown is substantially independent of the absolute value of the shield potential. By repetitive breakdown, it is intended to signify the breakdown without any appreciable time in terval after a prior extinguishment of the lamp and with out any change in the shield potential. Repetitive break down voltages were measured with many lamps and with the shield potential set at various voltages. In general, the breakdowns were found to be the same for nearly all shield potentials. Some lamps showed a slight increase in breakdown at high shield voltages, that is, at shield voltages substantially greater than the breakdown voltage of the lamp.

Whereas the breakdown does not vary with the absolute shield potential, our tests have established that a definite relation exists between a change or increment in shield potential made after the lamp is extinguished, and the breakdown of the lamp. With regard to any changes in shield potential made after a prior extinguishment, the lamp appears to record such changes; in other words, the lamp has a memory characteristic. The breakdown varies systematically with the magnitude of the change made since the prior extinguishment. On the other hand, the glow erases all memory, that is, any prior changes have no subsequent effect and any change made while the lamp is glowing has no effect on the next breakdown. Hereafter, the phrase, shield potential change, will refer to such a change made after extinguishment of a prior glow.

Referring to Fig. 5, curve 31 is typical of the break down variation with respect to shield potential changes. A negative change in potential raises the breakdown; a positive change lowers it. The breakdown is generally constant for the same magnitude of shield potential change between various absolute values. Thus, the breakdown for a shield change of -30 volts is the same whether the change occurs from +100 to volts, or from +30 volts to ground.

The breakdown vs. shield potential pattern of Fig. 5 is highly reproducible, and points taken in any order fall on the curve. Several groups of lamps with different combinations of gas composition, gas pressure and emission mix were tested. Although the average curve varies somewhat with the combination, the individual curves for lamps within any one group are quite similar and in general all groups follow the general behavior pattern shown in Fig. 5. It is desired to emphasize that this breakdown pattern is followed when there is substantially no rest time between successive breakdowns, that is when the interval between a prior glow and a succeeding breakdown is only few seconds. If an appreciable rest time is allowed, the breakdown may depart decidedly from the instant pattern.

Eficct of complex shield voltage changes When complex shield potential changes are made, within certain limits it is the net resulting change which determines the breakdown. If a negative shield change is made first and is followed by a positive change, it is the net difference considered as a simple shield potential change which governs. In other words, a subsequent pofitive change cancels a prior negative change volt for v0 t.

However, if the positive change is made first, the breakdown depends upon the net ditference only if the positive change is less than a critical value of about 35 volts. Up to this critical value, the cancellation is approximately linear; beyond it, there is a transitional region where only a decreasing fraction of a negative volt change is needed to nullify one positive volt change. Beyond this transitional region, the lamp appears unable to store or record any further positive changes and the breakdown voltage will be determined largely by the later negative change.

The above-described characteristics of the lamp appear to explain the variations in the breakdown observed as between diiferent persons handling and testing the same lamp. The person picking up the lamp puts on its envelope a definite potential which is usually different from the existing ambient potential and which alters the first breakdown as a result of the change. Since people vary in potential, the initial breakdowns obtained by different operators, or by the same operator on different days, are not consistent and have given rise to confusion.

sums and emission mixes.

After a lamp has been extinguished, the duration of the rest time to a subsequent breakdown has a very decided eifect on the breakdown voltage. The influence of rest time on breakdown may be studied with the circuit arrangement of Fig. 2, switch 24 being closed to contact 25, and shield potential changes being measured through voltmeter 27 with switch 28 closed. The pro cedure followed consists in glowing the lamp to erase all previous history, extinguishing the lamp, changing the shield potential by sliding tap 22, and, after different periods of time, finding the breakdown voltage as determined by voltmeter 21.

Fig. 6 shows the variation in breakdown potential with rest time. The rest time has been plotted on a logarithmic scale and curves 32, 33, 34, 35, and 36 are for shield potential changes AV equal to 90, l, 30, 0 and +45 volts, respectively. All shield potential changes Were made so that the final shield potential during the rest period was at ground level; thus, the AV was made from +30 volts above ground down to 0, and the +AV was made from 45 volts below ground up to 0.

The curves of Fig. 6, which represent average data for a considerable number of lamps, indicate that there is some condition within the lamps subject to a decay process. This condition holds for a few minutes and then gradually decays, reaching a final or terminal value in a maximum of 500 minutes. It appears as if a bucking potential were gradually put on the electrodes, having the same efiect as a negative shield potential change. The terminal or plateau breakdown value, is higher than the repetitive, that is the zero change-zero time breakdown,

and is equivalent to the breakdown immediately following a shield change AV equal to -30 volts. If the initial change in shieldpotential is greater than this equivalent change, the excess appears to leak off in approximately the same length of time; if the initial change is less, the deficiency is likewise made up in approximately the same length of time. Some experiments were continued for several thousand minutes,the longest for 33,000 minute (21 days) with no observable change in the terminal breakdown value.

In general, the curves of Fig. 6 appearto be fundamental for glow discharge lamps having cold activated electrodes; for instance, electrodesactivated with alkalineearth oxides. The patterns which they represent hold good for lamps with various gas compositions, gas pres- Lamps made up to the same specifications in respect of these variables appear in general to have substantially equal terminal breakdown values and decay time relative thereto. The patterns are more reproducible after the lamps have been stabilized, either .by high current aging for short periods or by low current aging for longer periods. The lamps which give the most consistent results are those which include a getter such as aluminum or magnesium, or both, included in an emission mix consisting in the main part of barium and strontium oxides.

Stabilization of breakdown independently of rest time To permit the use of glow discharge devices in dark enclosures as control elements in electronic circuits, it is necessary to stabilize the variation of breakdown with respect to rest time. In accordance with the invention, one arrangement for efiecting such stabilization is dependent upon the observed equivalency of the terminal breakdown to an immediate shield potential change AV equal to -30 volts. Thus, by applying to the shield a change of -30 volts immediately after extinguishment of a prior glow, the terminal breakdown value is immediately reached. At this level, there is no decay, as indicated by the fact that curve 34 in Fig. 6 remains horizontal at all times. Thus a constant breakdown value is effected which is independent of rest time.

A more convenient andprac'tical arrangement for ettecting a potential change on the shield is to connect the .shield potentiometer across the electrodes. Referring to Fig. 2, in order to 'efiect this arrangement switch 24 is closed to its lower position, that is, against contact 26. The setting of tap 22 on potentiometer 23 determines the proportion of the electrode voltage which is applied to the shield; switch 28 is preferably left open in order to prevent voltmeter 27 from unbalancing the proportionality set up by the position of the tap on the potentiometer. With this arrangement, as the electrode voltage is reduced to zero when a prior glow is extinguished and later raised to breakdown, there occurs automatically first a negative shield change and then a positive change. The negative change depends on the amount that the electrode voltage decreases after extinguishing; in other words, it is proportional to the maintaining potential of the lamp and this potential is substantially constant irrespective of all other factors. The positive change will be in proportion to thebreakdown potential, which for a given lamp is determined by the prior shield voltage change and 'by the rest time intervening since such change occurred.

Refering to Fig. 7, the variation in breakdown voltage with respect to time is shown for several ratios of shield voltage to electrode voltage. For convenience, the curves have again been plotted on a logarithmic time scale and the various curves 37, 38, 39, 40 and 41 are illustrative respectively of V /V ratios of 0%, 35%, 45%, 51% and it will be appreciated that the aforementioned ratios are determined by the ratios of resistance in the potentiometer on either side of tap 22, in the 0% case the tap being down at the grounded end.

it will be observed that there is a particular ratio, approximately 45% for the instant lamp, wherein the breakdown remains substantially constant with time, as

illustrated by horizontal curve 39. The ratio of V /V is that which gives an initial shield potential change AV equal to 30 volts, thereby producing a constant breakdown with respect to time. Thus for the instant lamp, the maintaining voltage being 66.7 volts, a V /V ratio of 45% gave an initial AV equal to 30 volts. However, it must be appreciated that the final breakdown voltage with this arrangement is lower than with an arrangement providing only a AV equal to -30 volts after a prior extinguishment. The reason for this is that there also occurs a positive shield potential change equal to 45% of the applied electrode voltage concurrently with the application of the breakdown potential. This positive change causes a lowering of the breakdown potential to that extent; however, this lowering is constant in amount, so that stable operation is achieved. it is for this reason that curve 39 representing the 45% ratio in Fig. 7 indicates a breakdown voltage of 91.4 volts, whereas curve 34 corresponding to a AV equal to 30 volts in Fig. 6 indicates a breakdown voltage of 103 volts.

Fig. 3 illustrates a practical circuit arrangement for applying to the shield 9 a constant proportion of the voltage applied across the electrodes. In this arrangement, cathode 3 is grounded and the positive input signal is applied to input terminal 43. Resistances 44 and 45 are connected in series between input terminal 43 and ground and the shield 9 is connected to their junction point. The relative values of resistors 44 and 45 are selected so that the proportion of the maintaining potential of the lamp which is applied to the shield as a negative change when the lamp is extinguished is equal to the desired critical AV for instance -30 volts for the instant lamp. It will be appreciated of course that the voltage dividing means connected between input terminal 43 and ground is not necessarily restricted to resistances; other impedances may be used for the same purpose. Where maintaining the leakage current to a minimum is of importance, a series capacitance divider should preferably be used.

Another arrangement in accordance with our invention for stabilizing the breakdown potential is based on our discovery that the terminal breakdown value may be shifted by maintaining a constant potential on the shield during the whole time interval while the lamp is extinguished. Referring to Fig. 6, it will be observed that the terminal voltage is higher than the breakdown voltage immediately following a AV equal to (curve 35). As previously mentioned, these curves hold good when the potential applied to the shield during the rest period is zero, that is when the shield is grounded. The extent to which the breakdown voltage rises during the rest period, that is the extent to which the terminal breakdown rises above the repetitive breakdown, may be controlled by applying a constant potential to the shield. This statement is not contradictory to our earlier statement that a constant potential on the shield has no eiiect on the breakdown voltage. The earlier statement was made with respect to the repetitive breakdown, that is, the breakdown very shortly after a prior extinguishment. The present statement is made with respect to the terminal breakdown, that is, the breakdown after the decay time interval.

Our tests indicate that the extent of the rise of. the terminal breakdown over the repetitive breakdown decreases with the application of a constant positive potential to the shield. if the potential applied to the shield is SUiTlClGIltlY high, the terminal breakdown may become lower than the repetitive breakdown. For the instant lamp which has been particularly considered in this application, my tests indicate that a constant potential of +43 volts applied to the shield will make the terminal breakdown substantially equal to the repetitive breakdown. Thus, by applying to the shield a positive potential equal to approximately 43 volts, constancy of breakdown may again be achieved.

A practical circuit arrangement for obtaining stability of breakdown by the application of a constant positive potential on the shield relative to the cathode is illustrated in Fig. 4. The cathode 3 of the lamp is grounded and the positive input signal is applied at terminal 46 to which the anode i is connected A constant positive potential relative to the cathode 3 or ground is applied to the shield 9 by means of a battery 4'7. With this arrangement, the breakdown potential of the lamp in its dark enclosure is stable at the repetitive breakdown value, namely the zero time, zero shield potential change value. For the instant lamp, this value is 94 volts as illustrated by curve 35 in Fig. 6.

Theory and method of operation The foregoing phenomena may be rationally explained on the basis of the following theory, which is based on the observed facts and which has been confirmed by numerous tests. it is to be understood, however, that while this theory is believed to offer the true explanation to the observed phenomena, the invention is not to be considered dependent on the validity of the theory.

In accordance with the theory, during the time when a. discharge is taking place in the glow lamp, there is a wall charge on the inside of the envelope. This charge is positive and approximately that of the anode. When the lamp is extinguished, the wall charge falls to a residual value of approximately +43 volts. This value appears to be that at which the gaseous filling in the lamp becomes totally tie-ionized and loses all conductivity. This value may vary with the constituents of the device and of the gaseous filling and its pressure. The residual wall charge will normally slowly leak off, because the enveope is made of a material such as glass which is not a perfect insulator. However, this residual wall charge does not of itself afiect the breakdown voltage. The breakdown voltage variation is governed by another charge induced at the electrodes, more particularly at the cathode which has the eflective charge. This charge is caused by a change in the surrounding electric field, and is stored in the emissive coating.

Under normal conditions, the residual wall charge gradually leaks off over a period of time. This leaking off produces an electric field change which induces a bucking voltage at the cathode which is stored in the emissive coating. This explains the gradual rise in the breakdown voltage from the repetitive breakdown to the terminal breakdown over a time interval of approximately e t he s. Apparently the stored charge in the emissive g at the cathode is stable at the equivalent terminal breakdown value and will not leak off.

The mechanism whereby the application of +43 volts to the shield stabilizes the breakdown voltage now becomes apparent. The voltage applied to the exterior of the glass envelope is equal to the voltage resulting from the internal wall charge: hence, the voltage gradient through the glass is zero and the internal wall charge cannot leak off. As a result, there is no charge induced and stored in the cathode, so that the breakdown voltage remains constant.

The mechanism by which the breakdown voltage may be stabilized at the terminal value by the immediate application of a 30 volt change to the shield appears to be as follows. The 30 volts change on the shield immediately induces at the cathode the maximum bucking voltage which can be stored in the emissive coating and remain stable without leaking off. Of course the internal wall charge on the envelope will then leak off and tend to augment the stored charge in the cathode coating. However, further charge in the cathode above the equivalent terminal value leaks off as fast as it is induced by the leaking off of the envelope wall charge. Hence the stored charge in the cathode remains fixed at the equivalent terminal breakdown value.

From the foregoing, it will be seen that our invention provides a new method of controlling the breakdown voltage in a glow discharge device of the type which has been described, which in its broadest aspect consists in regulating the stored charge in the emissive coating on the electrode. Where the object is to stabilize the dark breakdown voltage at a constant value, the method may be applied in either of two Ways. One mode of application consists in preventing the induction and storage of a charge in the emissive coating by preventing the leaking off of the charge on the envelope wall. This may be carried out by applying a predetermined constant potential equal to the internal wall charge (+43 volts for the instant lamp) to the exterior of the envelope wall, as for instance, to a shield surrounding it. The other mode of application consists in storing in the emissive coating, immediately upon extingnishment of the lamp, the maximum charge which can be stored without leaking off with time. This method may be carried out by applying a negative voltage change (30 volts for the instant lamp) immediately to the shield surrounding the lamp envelope. Since the stored charge on the emissive coating of the cathode is already at its maximum stable value, the subsequent leaking Off of the residual charge on the internal wall of the envelope is ineffective in further changing the breakdown voltage.

Conclusion While certain specific glow discharge device circuits it is to be understood that these embodiments are intended as illustrative and not as limitative of the invention. Even as the basic principles and method of operation, and previously unknown characteristics of glow discharge devices which have been disclosed herein may be used to stabilize the breakdown voltage, they may also be used to provide other predetermined or regulated breakdown patterns. Furthermore, it will readily be appreciated that in view of the slow decay characteristics of the effects of a potential change on the shield, the

instant glow discharge device arrangements may be used as memory devices to record electrical quantities. Other uses for the arrangements which I have disclosed will naturally occur to those skilled in the art. The appended claims are therefore intended to cover any modifications coming within the true spirit and scope of the invention.

What we claim as new and desire to secure by Letters Patent of the United States is:

1. In combination, a gaseous electric glow discharge device comprising an envelope containing an inert gas at a low pressure and a pair of cold electrodes coated with an electron-emissive mixture containing alkalineearth oxides sealed therein, a light-impervious enclosure surrounding said device, a conductive member surrounding said envelope and in electrical charge-imparting relation thereto, and means applying to said member, upon each extinguishment of said device, a negative voltage change of approximately 30 volts to effect immediately the terminal dark breakdown voltage of said device.

2. In combination, a gaseous electric glow discharge device comprising an envelope containing an inert gas at a low pressure and a pair of cold electrodes coated with an electron-emissive mixture containing alkalineearth oxides sealed therein, a light-impervious enclosure surrounding said device, a conductive member surrounding said envelope and in electrical charge-imparting relation thereto, and means automatically applying to said member, upon each extinguishment of said device, a negative voltage change of approximately 30 volts, said means comprising a voltage divider having its ends connected across said electrodes and having an intermediate connection to said member.

3. In combination, a gaseous electric glow discharge device comprising a vitreous envelope containing an inert starting gas at a pressure of approximately 130 millimeters of mercury and a pair of closely spaced metal wire electrodes coated with an electron-emissive coating containing barium and strontium oxides sealed therein, a light-impervious enclosure surrounding said device, and a conductive member surrounding said envelope and in electrical charge-imparting relation thereto, and means applying to said member a constant positive potential of approximately 43 volts relative to the one of said electrodes operating as a cathode whereby to make the breakdown voltage of said device substantially constant independent of rest period.

4. In combination a gaseous electric glow discharge device comprising a vitreous envelope containing an inert starting gas at low pressure and a pair of closely spaced metal wire electrodes coated with an electron-emissive coating containing barium and strontium oxides sealed therein, a light-impervious enclosure surrounding said device, and a conductive member surrounding said envelope and in electrical charge-imparting relation thereto, and means applying to said member a constant positive potential of approximately 43 volts relative to the one of said electrodes operating as a cathode whereby to make the breakdown voltage of said device substantially constant independent of rest period.

References Cited in the file of this patent UNITED STATES PATENTS 2,171,580 Macksoud Sept. 5, 1939 2,349,012 Spaeth May 16, 1944 2,487,437 Goldstein Nov. 8, 1949 

